1. Field of the Invention
The present invention relates to the measurement of the radiation characteristics of radio frequency (RF) antennas, and more particularly to a near-field measurement method which, while measuring the radiating near-field of an antenna under test (AUT), detects relative vibration between an AUT and a near-field measurement probe.
2. Description of the Background
High performance antennas are becoming increasingly prevalent as spacecraft, aircraft, ship, and ground vehicle mission requirements become more sophisticated. One problem in the development and manufacture of high performance antennas is the accurate measurement of antenna performance. Traditionally, antenna performance measurement was conducted by placing the antenna at a remote location, and measuring the amplitude response characteristics as a function of orientation of the antenna throughout its operational range. Required measurement distances for high gain antennas range from fifty feet to three miles or more. This measurement technique, known as far-field testing, suffers from significant practical limitations, such as susceptibility to the effects of weather, ground reflections, and increasing real estate costs.
As an alternative to far-field testing, near-field testing was developed. Near-field testing is conducted to determine an antenna""s performance or pattern, or to locate defective elements on the antenna. A near-field test consists of accurately passing a probe across the face, or a virtual surface, of an antenna. A typical near-field measurement system consists of three primary subsystems; a computer, a robotic positioner, and a probe. The computer provides the user interface and controls the operation of the probe. In addition it commands the robotic positioner which moves the antenna under test (AUT), the probe, or both over the desired virtual surface. However, relative vibration between the AUT and the near-field test probe causes inaccuracies/errors in the data collected.
Generally, in a near-field test, the probe is used to emit an RF signal that the AUT receives. The near-field testing equipment records the amplitude and phase of the RF signal received for a series of discrete data point across a virtual surface of the AUT. Typical testing protocol requires that the discrete data points be no farther apart than one-half the length of the RF signal""s wavelength. The discrete data points are usually collected along a plane located two to five wavelengths in front of the AUT, or along the surface of a cylinder or sphere that encloses the AUT. The system may also be operated in reverse with the AUT emitting and the probe receiving a series of RF signals. The computer is then utilized to transform the amplitude and phase at each discrete data point into a far-field pattern representing that for the AUT. The computer may also transform the data into the amplitude and phase at the face of the AUT (it is not possible to use a probe this close to the antennaxe2x80x94its presence affects the electric field to a degree that renders meaningless any data that is collected). By comparison with expected data, malfunctioning antenna elements are clearly identified.
Knowing the precise locations of the probe and the AUT when a discrete amplitude/phase data point is recorded is the most critical element of a near-field test. Relative movement (e.g. vibration) between the probe and the AUT may introduce inaccuracies/errors into the process. Typically, if displacement due to relative vibration exceeds 1/100 of the RF wavelength, accuracy of the near-field test will be reduced. Therefore, in order to minimize relative movement/vibration, near-field testing typically has been conducted in an indoor test facility utilizing fixed, rigid equipment with the AUT mounted on a stable, rigid fixture. However, this form of testing requires the presence of the antenna at the test facility. Depending on the location of the antenna to be tested, the cost of its transportation to and from the test facility, and the opportunity cost while the antenna is out of service, the indoor testing process is not always a cost-effective means of evaluating an antenna.
In response to the foregoing concerns, an array of portable testing equipment has been developed. However, the nature of on-site (i.e. outside of a test facility) testing using portable testing equipment generates a greater degree of inaccuracy/error in the measurement of the position of the probe and/or the AUT due to relative vibration. By its very nature portable near-field equipment cannot be as heavy and/or rigid as fixed, laboratory equipment. Consequently, the probe and the portable structure supporting/moving the probe are more likely to vibrate due to the surrounding environment (e.g. nearby ground vehicle vibrations, wind, inadvertent bumping of the portable structure). Similar forces may also cause the AUT to vibrate when it is mounted on a transportation device (e.g. a truck, a trailer, an aircraft) rather than a rigid test fixture. Any relative vibration between the probe and the AUT caused by the surrounding environment will cause test inaccuracies/errors. It is extremely rare where there is no relative vibration, or the probe and the AUT vibrate with exactly the same frequency, phase, and amplitude. In all other cases, and in all practicality, data inaccuracies/errors are introduced.
There have been attempts to solve data inaccuracies/errors due to other factors. For example, U.S. Pat. No. 5,419,631 to Slater discloses an apparatus and method to compensate for position inaccuracies/errors introduced by the thermal expansion/contraction of the probe and the AUT. However, there are no known methods for use in detecting relative vibration between a probe and AUT during near-field testing which do not require additional equipment.
It is, therefore, the primary object of the present invention to provide a method to detect relative vibration between a-probe and an AUT during near-field testing.
It is another object to measure and compensate for data inaccuracies/errors generated by relative vibration between a probe and an AUT during near-field testing.
It is still another object to provide a method to detect relative vibration between a probe and an AUT during near-field testing, and measure/compensate for the data inaccuracies/errors generated thereby, that is economical and does not require any equipment beyond that normally utilized for a near-field test.
In accordance with the above objects, the present invention is a method of detecting relative vibration between an AUT and a probe during any type of near-field testing. The method also provides the means to measure and compensate for any data inaccuracies/errors created by relative vibration while requiring no additional equipment, or hardware, other than that normally utilized in conducting a near-field test.
The present invention introduces a change in the data collection process during a near-field test. In contrast to the standard process of collecting data only at a series of discrete points, data is sampled regularly as the probe moves with a constant velocity across its data plane/measurement field or virtual surface. In accordance with the present method of data sampling constant, each principle data point (the series of points typically separated by typically no more than one-half the length of the RF signal""s wavelength) is sampled along with a series of leading and trailing xe2x80x9csecondaryxe2x80x9d data points. The phase at each secondary data point needs to be recorded. While the principle data points are, as in a standard near-field test, transformed by a computer to determine the AUT""s far-field pattern (or to identify malfunctioning antenna elements), analysis of the phase variation in the secondary data points reveals any relative vibration between the AUT and the probe.
At each secondary data point, the phase is measured for a constant frequency and beam position. The phase of a single frequency RF signal is recorded at each secondary data point. Any relative movement between the probe and the AUT causes the phase to change. Relative vibration between the probe and the AUT causes the phase to vary with a frequency equal to that of the relative vibration. The near-field equipment computer then performs a Fourier transform of the time varying phase to the frequency domain. This transform causes the vibration frequency to show up as spikes in a frequency domain plot.
The implementation of this method results in the detection of relative vibration between the RF probe and the AUT. It is a method that is superior to others involving additional hardware. Vibration sensors attached to the probe and/or the AUT may easily fail to detect the full amount of vibration. For example, a vibration sensor attached to a single point on the AUT may not provide an accurate measure of the vibration that is occurring in the AUT directly opposite the probe (this is especially true for large antennas).
This method eliminates a source of test error and increases the confidence that an accurate test has been conducted. The vibration information provided by this method may be used to actively compensate (in analysis or hardware) for any principle data point position inaccuracy/error caused by the vibration.